Updating the seismotectonic setting for the Gulf of Aqaba

The Gulf of Aqaba is known for its high seismic activity in Egypt and the Middle East. An inversion technique was applied to 113 earthquakes of magnitude 2.5 to 7.2 to distinct subsets of data based on tectonic regionalization to define the stress regime in the Gulf of Aqaba involving the Eilat basin, Aragonese basin, and Dakar basin. The stress inversion revealed two active stress patterns; an active strike-slip in the Eilat basin and a dominant extensional regime in the Dakar basin, whereas both strike-slip and extensional regimes coexist in the Aragonese basin. The stress pattern in the Eilat basin is consistent with the movement along the Dead Sea Transform Fault. In contrast, the extensional regime in the Dakar basin aligns with the extensional stress field throughout the northern Red Sea. The coexistence of two dominant types of stress regimes in the Aragonese basin is likely a result of the superposition of the two main neighbouring stress regimes: the strike-slip regime along the Gulf of Aqaba Dead Sea Transform Fault and the extensional stress state across the northern Red Sea. The orientations of the minimum principal stress in the three basins are almost similar, indicating ENE trending, nearly horizontal extension.

. Tectonic setting of the Dead Sea Transform Fault (DSTF) and Gulf of Aqaba (the area of study) the faults from 44 and the geological map from the Annals of the Geological Survey of Egypt. This map was created by using ArcGIS 10.3 software and basemap produced by 70  www.nature.com/scientificreports/ mechanism solutions in the Gulf of Aqaba revealed that the area is dominated by a normal stress field with a SHmax of 159°1 6 . The focal mechanism solutions in the Gulf of Aqaba are intensely studied by several authors [11][12][13]15,[17][18][19][20][21][22][23][24][25][26][27][28][29] . These studies revealed a variety of fault types, including normal faults, strike-slip faults, and oblique-slip faults, reflecting the complexity of the structure in the Gulf of Aqaba, These solutions demonstrated that the main fault trends in the Gulf of Aqaba are confined to the NNE-SSW, which primarily represents the left lateral strike-slip motion that is parallel to the Gulf while the NW-SE and the WNW-ESE fault trends represent the normal dipslip motion that is distributed along the Gulf basins margin.
This work aims to update the current focal mechanism catalogue for the Gulf of Aqaba earthquakes and to establish the current stress state relying on the stress inversion of the focal mechanism solutions, as well as to find the extent to which the effect of the stress field in the Red Sea manifests within the Gulf. Stress field investigation is of great importance for understanding the specific seismotectonics of the Gulf of Aqaba. For the current work, we chose to examine and evaluate the stress state for each basin in the Gulf (Eilat, Aragonese, and Dakar basins) in contrast to the previous studies that evaluated the stress field for the entire Gulf.

Geology and tectonic setting
The Red Sea and the Gulf of Aqaba constitute the plate boundaries that separate Africa-Nubia, Arabia, and Sinai. The Red Sea rift system undergoes continuous oceanic spreading; the most active part is the spreading centres, particularly in the south. The Levant tectonic model links the oceanic spreading centre in the Red Sea to the westward offset of the Anatolia fault via the Gulf of Aqaba-Dead Sea transform [30][31][32] . According to these authors, the early Miocene tectonic opening in the Gulf of Aden and the Red Sea pushed the Arabian plate northward, resulting in a sinistral displacement of 105 km along the Levant Rift 30,[33][34][35] observed that the offset along the pre-existing geologic features or pinning points revealed that overall slip corresponds to 107 km, with 62 km of this slip occurring through the Early Miocene to Pliocene, followed by 45 km occurring from post-Pliocene to Recent. Africa and Arabia are recently diverging across the southern Red Sea at a rate of 1.7 ± 0.1 cm/year in a NE-SW direction 36 while 2.4 cm/year according to 32 where suggested that there is an acceleration in the Red Sea opening resulted at least partially from the completion of the oceanic spreading centre along the length of the Gulf of Aden, decoupling the Arabia and Somalia plates. In the northern Red Sea, the opening rate reduces to about ~ 1 cm/year 37 . The Plate reconstructions suggest that these opening rates must have been about half these values before 11 ± 2 Ma 38 . This opening led to the generation of NNW-SSE oriented Gulf of Suez continental rift (the western arm of the Red Sea), the Red Sea, and the NNE-SSW trending Gulf of Aqaba transform. Delaunay et al. 39 Shows the main difference between the southern and northern Red Sea lies in the direction and rate of plate motion as a result of the geodetic constraints given by the Euler pole of rotation to the north. Consequently, the ca. 30° counterclockwise strike change and halving of the spreading rate (ca. 16 to ca. 10 mm. year −1 )between 18°N and the Suez triple junction result in a shift from slow, orthogonal to oblique.
The opening of the Red Sea and Gulf of Suez began in the late Oligocene and continued thereafter, however, the movement of the Dead Sea Transform has replaced the extension in the Suez Rift and allowed the ongoing opening of the Red Sea Basin 40 , which may have occurred at the end of Miocene simultaneously with the opening of the Gulf of Aqaba 41 . The Gulf of Aqaba represents a transition stage from the spreading zone in the Red Sea to the DSTF which is characterized by left-lateral strike-slip displacement 35,42 was one of the first researchers to identify this fault zone and its associated sense of motion. The Gulf of Aqaba is a tectonically active region that forms an "echelon" strike-slip fault system along the plate boundary between Nubia-Sinai and Arabia. It occupies the southernmost part of the Dead Sea Transform Fault (DSTF) and also has three main pull-apart basins which developed between the overlapping ends of a major left-lateral strike-slip fault. Eilat Deep, Aragonese Deep, and Dakar Deep are the three pull-apart basins 43 . Based on multibeam bathymetry 44 , the Gulf of Aqaba can be categorized into six basins, including the Dakar Deep, Tiran Deep, Hume Deep, Aragonese Deep, Arnona Deep, and Eilat Deep Fig. 1. The two basins in the northern part of the Gulf of Aqaba, the Eilat Deep and the Aragonese Deep are well separated and exhibit a typical pull-apart morphology, according to multibeam bathymetric data. The smaller Arnona Deep is situated close to the Egyptian coast, southwest of the Aragonese Deep. Further south, the Dakar and Tiran Deeps are surrounded by a common series of faults despite being morphologically distinct and separated by just a small high. The Eilat Basin is bordered by strike-slip and normal fault systems, with the first group representing the Eilat and Aragonese faults, which border the basin from the west and east, respectively. The second group belongs to normal faults that run along the basin's northern and southern limits. The Aragonese Basin is also bordered by strike-slip and normal fault systems. Strike-slip faults include those that border the Aragonese basin from the west and the east respectively, and are known as the Aragonese and Arnona faults. The basin is surrounded on the north and south by NW-trending lines, which constitute the normal faulting. A smaller secondary basin called the Arnona Basin exists at the southern end of the Aragonese fault, to the southwest. This Basin is isolated from the Aragonese basin by an elevated sea floor, flanked by normal faults on each side and bordered on the west by oblique-slip faults. At its southern extreme, the Dakar fault, which defines the southernmost basins, the Dakar and Tiran, exhibits dip-slip normal motion. In these basins, there is no evidence of strike-slip motion. The Tiran Basin's southern boundary is marked by a set of NW-trending short parallel normal faults.
On the longer geological timescale, the DSTF extends over 1200 km from the southernmost end of the Gulf of Aqaba and links extensional tectonics in the Red Sea to contraction tectonics in the Zagros-Bitlis convergence zone of eastern Turkey 34,42,45 where it forms the northern part of the Syrian-African rift system. The largest earthquakes in the Gulf of Aqaba region are categorized as shallow earthquakes. Their depths did not exceed 15 km, indicating that these earthquakes occurred in the Gulf of Aqaba's upper crust, which is marked by a continuous vertical transition from brittle deformation near the surface to ductile deformation at lower www.nature.com/scientificreports/ crustal depths 46 . El-Isa 47 reported that the majority of small magnitude earthquakes in the Gulf of Aqaba occur in the upper crust at depths shallower than 20-22 km, with a significant majority occurring at depths of 15 km. This pattern similarly reveals a brittle upper crust and a ductile lower crust. These earthquakes do not reach the Moho because the depth down to the Moho discontinuity in the Gulf of Aqaba thins from the north (35-37 km) to the south (27-28 km), suggesting a southward increase in extension towards the Red Sea, which likely governs the structural history of the southern part of the gulf 48,49 .
Global positioning system (GPS) observations revealed that the present left lateral displacement rates along the DST fault are currently around 5 mm/year [50][51][52][53] . while geological evidence indicated faster rates of long-term displacements between 5 and 10 mm per year in the past starting from the initiation of the DST 20-15 Ma ago 34,54 . Methodologies of focal mechanism solutions and stress tensor inversion. We compiled all available data of earthquakes presented in this study from different sources, among which (1) the focal mechanism solutions published in various literature between 1982 and 2011; (2) polarities of P waves first onset, amplitudes of S waves and S/P amplitude ratio between 2012 and 2021 55 . The polarity and amplitude data were extracted from the digital waveforms through several local and regional agencies to increase station numbers and reduce the azimuth gap as much as possible. These data were obtained from the databases of the following sources In the current study, the new focal mechanism solutions were constructed for 9 events with 3.6 ≤ ML ≤ 4.2, covering the period from 2012 to 2021. The list of these earthquakes is shown in Table 1 and their fault plane solutions parameters in Table 2.
The initial focal mechanism solution for the new 9 earthquakes was constructed from the first P-wave polarity using PMAN software of 56 . Subsequently, the focal mechanism solutions were recalculated using focmec software 57 . This software calculates the focal mechanism solutions depending on the polarity of the first P-wave onset, polarities of S H and S V phases, and the amplitude ratios of (S H /P) , (S V /P), and (S V /S H ). The first-motion amplitude data (S V /P, S H /P, and S V /S H ) contribute to improving the identified solutions. FOCMEC conducts a grid search over all possible solutions based on the user-selected parameters, including polarity errors, the range of disagreement between the observed and calculated amplitudes, and the number of ratio errors that are permitted outside a particular range. The corresponding amplitude ratio error is calculated according to the www.nature.com/scientificreports/ maximum allowed log 10 ratio 58 . Our solutions are estimated using a 5° grid search while the maximum allowable log 10 ratio is 0.6. The polarities and amplitudes of the P-phase were picked from vertical components seismograms, while S V and S H polarities were picked from the radial and transverse components generated by the rotation of the two horizontal components using the seismic analysis code (SAC). Before making rotation some procedures must be applied, including (1) instrumental correction for selected three components stations (2) calculating spectrogram to detect the exact duration of different phases to avoid interference and contamination between phases as shown in Fig. S2 (supplementary material) (3) rotation of the two horizontal components to obtain radial and transverse components of S-phase. Following the rotation, various spectral analysis procedures are employed to get the corrected spectrum, as explained in Fig. 2. Finally, the corrected observed spectrum of the earthquake was fitted to the theoretical curve of the Brune source model 59 . This fitting offers the spectral amplitude of the flat part Ω• of the different phases. Once we have obtained Ω• of the various phases, as illustrated in Fig. 3, we then calculate different amplitude ratios such S V /P , S H /P and S V /S H . Figure 4 shows the spatial distribution of the new focal mechanism solutions that we have constructed for the nine earthquakes added by this study. The focal mechanism solutions for these events are shown in Fig. 5 and the details (are shown in Fig. S1 in the supplementary material).
In addition, we computed the seismic moment and energy released for the various types of mechanisms involved in the nine earthquakes that occurred in the Gulf of Aqaba between 2012 and 2021. We divided the mechanisms into strike-slip and normal mechanisms as a result, and we estimated the total seismic moment and energy released in each basin as well as for the whole Gulf (Table 3).
In the current study, we applied the stress tensor inversion technique of 6 to evaluate the stress field for each basin in the Gulf to find out whether the Gulf is influenced by the Red Sea opening or if it is tectonically linked to movement along the DSTF, using the focal mechanism database constructed in this work. In contrast to our work, earlier research examined the Gulf 's overall stress status.
Various approaches for performing the inversion have been proposed 5,60-62 developed one of the most common stress inversion techniques; modifications and extensions were proposed by 9,60 . These methods assume that (1) the tectonic stress is uniform (homogeneous) in the region (2) earthquakes occur on pre-existing faults with varying orientations (3) These inversion methods are based on the Wallace-Bott hypothesis, which assumes that slipping (d) on a fault surface occurs in the direction of maximum shear stress (τ) and it applies to both newly www.nature.com/scientificreports/ formed faults 63 and reactivated ones. The angle between (τ) and (d) is the misfit angle (α) which should be minimized for each earthquake (i). Angelier 64 identified four independent parameters that represent the orientation of the reduced stress tensor. These parameters include σ, σ 2 , σ 3 and the ratio of the principal stress difference R. The stress ratio, R, defines the geometry of slip-along fault planes and governs the orientation of shear stress for any particular plane 63 . In the current study, we used the stress tensor software (TENSOR software) of 6 for identifying the four parameters of the reduced stress tensor. This software does not require a prior decision regarding which of the two nodal planes to use before inversion. In the initial stage of the processing, the four parameters of the reduced stress tensor are estimated roughly using the improved Right Dihedron Method, which is based on 65 work. Additionally, this technique eliminates focal mechanisms that are incompatible with the predominant data set. The filtered focal mechanisms will be used as the first step in the Rotational Optimization inversion technique. The Rotational Optimization inversion technique employs an iterative grid-search of stress tensors to minimize the angle deviation (α) between the modelled and observed slip lines on the plane, preferring a higher shear stress magnitude ‫|‬τ(i) ‫|‬ and lower normal stress ‫|‬v(i) ‫|‬magnitude. The Tensor software employs F5 misfit function while 6 employs F3. The Rotational Optimization inversion technique initially inverted both nodal planes; the nodal plane that best fits the uniform stress field would be chosen as the actual fault plane 61 . The orientations of the horizontal stress axes (SHmax and Shmin) are computed with the formula of 66 . The stress regime index R′ and the stress ratio R values are used to define and identify the stress regime, as follows: In this study, we derived the stress from the focal mechanism solutions database for the earthquakes which occurred in the vicinity of the Gulf of Aqaba. These earthquakes are listed in Table 4, and their focal mechanism parameters are listed in Table 5. The stress tensor inversion was derived for each basin in the Gulf, including the Dakar basin in the south, the Aragonese basin in the central, and finally the Eilat basin in the northern part of the Gulf. The results of stress inversion are evaluated according to the quality ranking starting from A to D. This quality was updated using the quality ranking system of the World Stress Map Release 2008 6 where:  www.nature.com/scientificreports/ Before computing the stress tensor using the focal mechanism solutions, we first validated the solution's degree of homogeneity and similarity by calculating the Kagan angle 67 . Kagan angle is a measure of the differences between the orientations of two fault planes in two different focal mechanism solutions. It detects and evaluates the minimum rotation angle between two source mechanisms. The Kagan angle varies between 0° (for identical solutions and full agreement between the two solutions) and120° (for absolute inconsistency and total conflict), the Kagan angle below 60° indicates a good correspondence while above 60° means a mismatch 68 . According to 69 the pairs of solutions with an angle below 20°-30° were regarded as being very similar, while a Kagan angle of 60° is still considered as matching. The solutions that displayed a Kagan angle greater than 60° are excluded from this study. The results of the Kagan angle for the solutions that were selected in the Dakar basin ranges between 16° and 47° the majority of angles in the thirties. Out of the total of 21 solutions in this zone, we excluded 5 of them. The Kagan angle in the Aragonese basin ranges from 14.9° to 36.5° for normal solutions, with the majority falling in the twenties, and from 36° to 47.9° for strike-slip solutions, with the most falling in the forties. There are 50 total solutions in this basin; however, 20 of them are excluded. In the Eilat basin, the Kagan angle ranges from 10.14° to 44° with the majority in the thirties Out of a total of 25 solutions, 10 are excluded.

Results
Results of focal mechanism solutions. The results of 113 focal mechanism solutions show diversity in solutions where some of the solutions give pure normal faults, some solutions give left-lateral strike-slip faults and the other solutions give oblique normal faults. This is clear in the ternary diagram given in Fig. 6 which reflects the complexity of the geological and tectonic setting of the Gulf. These solutions clarified that the nodal planes follow the NNW-SSE, NW-SE, and NNE-SSW trends with different dip directions as shown in (Fig. S3) (Supplementary Material) (only some of these planes represent the actual trends for faults in the Gulf of Aqaba so we decided to calculate the stress tensor inversion to detect the actual trends in the Gulf). The beachball diagrams of 113 focal mechanism solutions in the Gulf of Aqaba are shown in Fig. 7A, and their hypocentral information and focal mechanism parameters are listed in Tables 4 and 5. Figure 7B shows the epicentres distributions for the available solutions in the Gulf. This distribution demonstrates the presence of the normal faulting www.nature.com/scientificreports/ mechanism throughout the Gulf. If we look into each basin individually, for instance, the Eilat basin, we will see that the Normal and the Strike-slip faulting mechanisms are somewhat equal. In contrast, the Aragonese basin contains three different types of mechanisms, but the Normal faulting and oblique normal faulting mechanisms predominate while the Dakar basin exhibits both oblique normal and normal faulting mechanisms. According to calculations of the total seismic moment and energy released by each type of mechanism for each basin and the entire Gulf, the seismic moment and energy produced by the strike-slip mechanism were higher than those released as a result of the normal mechanism.
The results of stress tensor inversion in the Gulf of Aqaba. The inversion technique described above was applied to a data set of 40 focal mechanism solutions in the Gulf of Aqaba. These solutions were subdivided into three different tectonic sub-regions including Dakar, Aragonese, and Eilat basins (Fig. 8). The inversion results of each sub-region will be discussed as follows:  Table 6.
Aragonese basin stress tensor. A reasonable examination of the focal mechanism parameters in the case of the Aragonese basin led to the separation of them into two dominating types. The first type involves 10 focal mechanism solutions. The best-fitting stress tensor model favours an extensional normal faulting regime with R′ of 0.5 which is supported by sub-vertical σ 1 and subhorizontal σ 2 axes with 83 and 05 plunges (Fig. 9B), respectively. The quality of the derived stress tensor is A, with a low misfit angle (α) of 10.1 and an average misfit function (F5) of 4.7 as shown in Table 6. The second type includes 11 focal mechanism solutions. The corresponding bestfitting stress tensor model indicates that the state of stress is dominated by horizontally to sub horizontally σ 1 and σ 3 Plunge 23 and 3, respectively while σ 2 is close to vertical ( Table 6, Fig. 9C). These findings manifest strike-slip regime with an A quality stress tensor, a low misfit angle (α) of 11.5 and an average misfit function (F5) is 5, This regime is characterized distinguished by N72 E extensional direction and a stress regime R′ of 1.59.
Eilat basin stress tensor. The best-fitted stress tensor model obtained from a subset of 11 focal mechanism solutions in Eilat the basin indicates that the state of stress is dominated by horizontally to sub horizontally σ 1 and σ 3 Plunge 15 and 9, respectively while σ 2 is close to vertical (72) as shown in Table 6 and Fig. 9D. These findings manifest a strike-slip regime with a B-quality stress tensor, a low misfit angle (α) of 13.5, and an average misfit function (F5) of 2.7. This regime is distinguished by N69 E extensional direction and a stress regime R′ of 1.25.

Conclusions
The Gulf of Aqaba which occupies the southern end of the Dead Sea Transform Fault (DSTF) is the present-day most active tectonic zone in Egypt (as well as on the DSTF as a whole). This significant transform fault, which forms the plate boundary between Arabia and Africa-Sinai/Levant, is a North-South trending left-lateral strikeslip fault. During this study, we constructed fault mechanism solutions (FMS) for the earthquakes that occurred between 2012 and 2021 with local magnitude starting from 3.5 for updating and completing the database of fault plane solutions of earthquakes in the Gulf of Aqaba. These solutions clarified that the Gulf is characterized by the presence of different types of mechanisms such as normal faulting, strike-slip faulting, and oblique normal faulting. The database of the focal mechanism solutions has been separated into groups based on tectonic regionalization, including Eilat, Aragonese, and Dakar basins to evaluate the temporal changes of the stress field www.nature.com/scientificreports/ in these basins. The distribution of fault plane solutions demonstrated that normal faulting mainly affected the southern part of the Gulf (the Dakar basin), three different types of mechanisms affected the Aragonese basin, and a strike-slip mechanism affected the Eilat basin. The seismic moment and energy released by the strike-slip mechanism in the Gulf of Aqaba are greater than those released by the normal faulting mechanism, demonstrating the significant effect of the strike-slip movement along the Dead Sea Transform Fault. By employing the inversion approach developed by 6 , which aims to select the actual nodal plane that most closely resembles the homogeneous stress field while calculating the stress tensor, it has been possible to determine the average stress field acting on each basin from the focal mechanisms of earthquakes from that basin. An additional analysis of the focal mechanism data that is currently available for the Aragonese basin, involving the separation of various stress states, has revealed the existence of a second stress state. The spatial variations of the stress field were analyzed based on the focal mechanism solutions with ML ≥ 2.5. The method developed by 67 was applied to confirm the homogeneity of the focal mechanism data for each basin prior to the implementation of the inversion, and as we mentioned earlier, a Kagan angle of 60° is still regarded as matching. Kagan angle results showed that for the Eilat Aragonese and Dakar basins, respectively, 60% (the percentage of fault planes that satisfied the conditions of Kagan angle), 66.6%, and 76.19% of fault planes satisfied the requirements of Kagan angle within the thirties Degree. www.nature.com/scientificreports/ The results of our analysis showed that the stress field in the Dakar basins is different from is not one would expect from the Dead Sea Transform Fault. The stress field in this basin displays an ENE-WSW extensional stress regime with the maximum compressional principal stress axis (σ1) being sub-vertical and the minimum extensional principal stress axis (σ3) being nearly horizontal. This stress pattern clearly illustrates how the Dakar basin has been affected by the incipient oceanic spreading in the northern Red Sea. The identified direction of extensional stress in the Dakar basin is nearly compatible with the direction of extension in the northern Red Sea. Our stress inversion for the Aragonese basin reveals two dominating stress patterns, a strike-slip regime, and an extensional regime. Subhorizontal orientation of both σ1 and σ3 with WNW and ENE trends, respectively are recognizable in the strike-slip regime. The second stress state is an extensional regime, which reflects ENE-WSW direction of extension. With a sub-vertical plunge of σ1 and a subhorizontal plunge of σ3. The orientations of σ3 in the Dakar and Aragonese basins are nearly similar to the direction of extension in the northern Red Sea, in conformity with the results of 14 based on the fault slip direction data in the southern part (Dakar basin) of the Gulf. The existence of two stress regimes in the Aragonese basins reflects the interplay between the incipient spreading centre in the northern Red Sea and the strike-slip motion along the Aqaba Dead Sea Transform Fault. The best fitting solution for the Eilat basin showed a strike-slip regime with subhorizontal axes for both σ1 and σ3, with WNW and ENE trends, which fitted to the regional stress field along the Aqaba-Dead Sea transform fault. www.nature.com/scientificreports/ A difficulty arises when comparing our results to those obtained from previous studies because our inversion is performed separately for three subsets of focal solutions that were categorized based on the tectonic regionalization of the Gulf of Aqaba, whereas previous studies inverted the solutions for the whole Gulf without any categorization 11,13,15,16 , or with a different categorization scheme 12 .
Finally, we may draw the following conclusion from studying FMS and stress tensor inversion in the Gulf of Aqaba: the tectonic setting in the Gulf of Aqaba is quite complex. Not just the DSTF's movement, but also the  www.nature.com/scientificreports/ movement in the Red Sea, has had a significant impact on it. While the movement along the Dead Sea Transform Fault continues to have a significant impact on the Eilat basin, the Red Sea's influence extends totally to the Dakar basin and partially to Aragonese basin which is still significantly affected by the Dead Sea Transform Fault strike-slip movement. The seismic moment and energy released resulting from the strike-slip mechanism are larger than those released from the normal mechanism. www.nature.com/scientificreports/

Data availability
The datasets used during the current study are available from the corresponding author upon reasonable request.  www.nature.com/scientificreports/